Zeolite catalysts have become widely-used in the processing of petroleum and in the production of various petrochemicals. Reactions such as cracking, hydrocracking, catalytic dewaxing, alkylation, dealkylation, transalkylation, isomerization, polymerization, addition, disproportionation and other acid-catalyzed reactions may be performed with the aid of these catalysts. Both natural and synthetic zeolites are known to be active for the reactions described above.
Early synthetic zeolites such as Zeolite A, X and Y and several mineral zeolites, chabazite, erionite, and mordenite, contain high silica/alumina ratios. Early laboratory research established that hydrolytic attack on framework aluminum atoms would cause loss of structural integrity (loss of crystallinity) of these zeolites. Additionally, the amount of alumina present appears directly related to acidity characteristics of zeolites. A correlation exists between the high acidity of such catalyst and the formation of coke in catalytic hydrocarbon processing using such catalysts. Coke formation increases the aging and reduces the stability of the catalysts. In order to overcome the deficiencies of the early zeolites, research was conducted to synthesize new zeolite structures having low aluminum atom content. Among the first successes was the production of Zeolites ZK-4 and ZK-5. Another success was the discovery of Zeolite ZSM-5, disclosed and claimed in U.S. Pat. No. 3,702,886. ZSM-5 catalysts are characterized by frameworks primarily composed of SiO.sub.2, with occasional aluminum substitutions. These catalysts are more particularly identified by an X-ray diffraction pattern disclosed in U.S. Pat. Nos. 3,702,886, the disclosure of which is incorporated by reference herein.
Generally, ZSM-5 catalysts are prepared from reaction mixtures comprising sources of silica, alumina, alkali metals, water, and a template composed of an organic nitrogen-containing cation such as tetrapropylammonium hydroxide (TPAOH). A gel or sol prepared by mixing the above ingredients is digested in a temperature range of 150.degree. C. to 200.degree. C. for about 25 to 190 hours to form a crystalline zeolite containing framework silica and alumina having a silica:alumina ratio of at least 12. The ratio can exceed 10,000. The product is then heated to remove water and activated by replacing its sodium cations with non-metallic cations, such as hydrogen and ammonium, by cation-exchange.
Because of their shape selectivity and exceptionally high degree of thermal stability, ZSM-5 catalysts find use in a host of hydrocarbon conversion reactions. But, because of their low aluminum content, their catalytic activity is not as great as other zeolites having a higher aluminum content. To improve the catalytic activity of ZSM-5 catalysts, the catalyst can be prepared so that it contains a framework metal other than aluminum. It also is advantageous, at times, to synthesize a catalyst so that its activity is directed to a specific hydrocarbon reaction. The addition of non-framework metals sometimes has this effect. U.S. Pat. No. 3,702,886 discloses a method for producing a catalyst containing framework gallium, a [Ga]ZSM-5 catalyst. However, the product produced by the method of the '886 patent contains a quantity of non-framework gallium which is not suitable for purposes of this invention.
The present inventors have developed two methods for preparing a [Ga]ZSM-5 catalyst. The catalysts produced by the methods of the claimed invention contain gallium, and the gallium is primarily present in the framework of the catalyst, and the catalyst contains little or no non-framework gallium.